Unexpected behavior when an attribute is inherited with AllowMultiple = false

Let's say I have the following attributes:

[AttributeUsage(AttributeTargets.Method, AllowMultiple = false, Inherited = true)]
class TestAttribute : Attribute { }

class FooAttribute : TestAttribute { }    
class BarAttribute : TestAttribute { }

I expect the attributes inherited from TestAttribute can't be duplicated for methods:

[Foo][Bar] //I expect compile error here
void M() { } 

But in fact the AllowMultiple check is only applied in the same attribute type, means compile errors happen for [Test][Test],[Foo][Foo],[Bar][Bar], but no error for [Test][Foo],[Foo][Bar].

It looks like by design than a bug? But with the definition of TestAttribute someone can easily make mistake like this:

//bang! why? AllowMultiple=false!
var testAttribute = someMethod.GetCustomAttribute<TestAttribute>();  
//or someMethod.GetCustomAttributes<TestAttribute>().SingleOrDefault(); 

My questions are:

Q1: Why do they design it like this? I think my expected behavior is more "correct"?

Q2: Any workaround? I don't want to put Foo/Bar things to an enum and use something like [Test(Flag=Foo)], because the inherited attributes have their different properties, putting them into one attribute is hard to maintain.